They got together during a recording session in late 1973, Iommi told Loudersound, and Led Zeppelin drummer John Bonham had a specific song from 1972's Vol. 4 in mind. Instead, Sabbath's guitarist said everyone just meandered around musically, slightly derailing the completion of the follow-up, Sabbath Bloody Sabbath.

"John wanted to play 'Supernaut,' but we jammed instead," Iommi said. "We were in the middle of recording, so it fucked up the session."

This was something of a theme with Bonham, actually. In fact, the ever-aggressive drummer had a history of damaging his Sabbath counterpart Bill Ward's kit.

"When we were playing clubs, John would sometimes come along and he’d want to get up and jam," Iommi said years ago. "The first time we said, 'Okay then.' So, he got up and played Bill's drums and just wrecked them. Bill was really pissed off, so after that anytime John came along and said, 'Can I have a go?,' Bill would go, 'No' and not let him play."

Ward was understandably concerned when Led Zeppelin showed up again at their studio – and rightfully so. "It escalated to a pretty crazy situation within about 30 minutes, because not only was Bonzo there, but Robert Plant and John Paul Jones were there as well," Ward told Rock Cellar in 2011. "Jimmy [Page] wasn't there, but I wish he had been. And Bonzo was kickin' the crap out of my drum kit!"

Ozzy Osbourne later admitted that he barely remembers this historic meeting – but that has nothing to do with the relative quality of their output. "I just jammed out, you know?" Osbourne told Rolling Stone in 2016. "I can't think back on it now, like, 'Whoa, we just jammed with Zeppelin.' It’s just what we did. We were all stoned anyway."

Turns out, much to Osbourne's apparent dismay, Zeppelin had a business-related motive for stopping by. "We were really good mates with Led Zeppelin – especially Robert Plant and John Bonham, who came from the Midlands," Iommi told Loudersound. "Zeppelin had wanted us to be on their label Swan Song, but we couldn't make it work out."

The issue of whether tapes of these mythical sounds actually exist seemed to have been settled when Ward told Back Page in 2011 that they did not. (Earlier speculation had their jam – which is sometimes dubbed "Black Zeppelin" – taking place during sessions for 1975's Sabotage, as well.)

"There was a moment during that jam where we all kind of got this crazy notion and said, 'Let's put something down on tape – but nothing transpired and no tape rolled," Ward said. "Nothing was recorded: We were just pissing about. I believe at one point Geezer Butler and Robert did a bit of writing together – but that was their own personal thing, between them. The Black Zeppelin recordings didn't ever exist."

Iommi, however, is now disagreeing. "I know that it was recorded, and I'd love to hear it," Iommi said. "The tape must be around somewhere."